Google Drive access tokens have a lifespan of one hour and Google Drive refresh tokens have a lifespan of a month. We expect the access tokens to expire, even during the course of a backup, and do check for that. In fact, we're using the refresh tokens to get fresh access tokens.
If the refresh tokens have expired there is no solution other than reauthenticating with Google Drive. That's how Alphabet has designed their Google Drive service; refresh tokens expire after about a month to prevent "inactive" refresh tokens leaked to malicious actors from being used to gain access to your files. Their whole OAuth2 authentication is based on the premise that tokens are going to be used "regularly" to access files in your Google Drive.
The problem is that you want to only run this once every 6 months. There is really no way to do that with Google Drive. We cannot instruct Google Drive to deliver "permanent" refresh tokens, or at least control the expiration date of the refresh tokens.
Even if you would use a third party service which exposes Google Drive over WebDAV (so you can use Akeeba Backup's Upload to WebDAV feature instead) you'd still run into the same problem if it's only used once every six months: the refresh token acquired by the 3rd party service would still expire in the meantime. As I said, this is a Google Drive limitation, not a limitation imposed by any software or service connecting to Google Drive. Likewise, if you had a computer with the Google Drive client and left it dormant for over a month you'd find that you still have to reauthenticate to Google Drive upon booting it after 1 month or longer for the exact same reason, which is to say that even Google Drive's own software is hit by the same limitation.
You can either back up those sites much more frequently (every 30 days at most) to keep the refresh tokens active OR you can use something which does not use OAuth2 authentication such as Amazon S3, Backblaze B2, or even an Internet-connected NAS at your office.
I would recommend taking bi-weekly (once every 2 weeks) backups and only keep the last 2 backups by enabling the Remote Quotas feature in Akeeba Backup and setting the Count Quotas setting to 2. This way you will have a recent backup (which means in case the site breaks you won't have to install 6 months worth of WordPress and plugin updates) and it will keep the Google Drive refresh token "active".
Nicholas K. Dionysopoulos
Lead Developer and Director
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